1. Field of the Invention
The present invention relates to a photocatalyst inducing oxidation and reduction under visible light, and more specifically to a photocatalyst that is highly activated, visible light induced, highly oxidative, and highly reductive.
2. Description of the Related Art
Atoms of a semiconductor are bonded by covalent bonding with a bandgap between valence and conduction bands. When the semiconductor is irradiated and the energy of the incident beam equals that of the bandgap or greater, electrons in the valence band are excited and transit to the conduction band, leaving holes in the valence band, thereby forming electron-hole pairs. Thus, the semiconductor can be a photocatalyst, wherein the excited electrons may be captured by electron acceptors of adsorption molecules to reduce the adsorption molecules, and/or the holes may capture electrons of adsorption molecules to oxidize the adsorption molecules in the lifetime of the electron-hole pairs. The redox potential of the adsorption molecules and potentials of valence band and conduction band of the photocatalyst determine whether and how the redox reaction occurs.
Fujishima et al. disclose a wet type photocell with a TiO2 electrode oxidizing water and generating oxygen under light irradiation in Japanese Patent Issue No.0633127. TiO2 becomes the most popular photocatalyst resulting from the properties thereof such as low water-solubility, high stability, non-toxicity, and low cost after this disclosure. The oxidation of water in the surface of photocatalyst such as TiO2 is shown in subsequent reactions.

The final product, hydroxyl free radicals (.OH), can react with and remove toxic contaminants.
However, bandgap of commercially used TiO2, Degussa P-25 type, is 3.2 eV, resulting a requirement for irradiation under light with energy of 3.2 eV or greater to form electron-hole pairs to induce the redox reaction. Wavelength of light with energy of 3.2 eV or greater is 387 nm or less, in the UV range. In solar energy normally received at sea level, only 5% is UV, 45% is visible light (wavelength between 400 nm and 800 nm), and 50% is IR. Thus, a visible light-induced photocatalyst is required to use the received solar energy effectively for photocatalyst application in low light environments such as in cars and indoors.
Anpo et al. disclose a photocatalyst of doped ionized metal, such as vanadium or chromium, in a TiO2 photocatalyst as disclosed in Japanese Patent Publication No.JP 9262482. The dopants replace parts of titanium ions in the TiO2 crystals, inducing energy bands lower than the conduction band of TiO2, thereby lowering the bandgap of the metal-doped TiO2 photocatalyst to approximately 2.0 eV. Thus, the electrons of the metal-doped TiO2 photocatalyst can be excited by irradiation using light having approximately 620 nm wavelength of visible light to form electron-hole pairs.
Morikawa et al. disclose a photocatalyst of doped ionized nitrogen in a TiO2 photocatalyst as disclosed in Japanese Patent Publication No.JP 2001205103. The dopants replace parts of oxygen ions in the TiO2 crystals, implementing Ti—O—N bonding and inducing energy bands higher than the valance band of TiO2, thereby lowering the bandgap of the nitrogen-doped TiO2 photocatalyst to approximately 2.4 eV. Thus, the electrons of the nitrogen-doped TiO2 photocatalyst can be excited by irradiation using light having approximately 520 nm wavelength of visible light to form electron-hole pairs.
However, TiO2 photocatalyst has greater bandgap, sufficient to offer electron-hole pairs respectively with an electron in high reduction potential (−0.2 eV, when that of H2/H2O is 0) and hole in high oxidation potential (+2.8 eV, when that of H2/H2O is 0) after irradiation, thereby providing TiO2 photocatalyst with high redox capability.
Although semiconductors with bandgap between 2.0 eV and 3.0 eV, such as GaP, GaAs, CdS, CdSe, WO3, Fe2O3, metal-doped doped TiO2, and nitrogen-doped TiO2 can form electron-hole pairs therein after irradiation using visible light as photocatalysts, electron-hole pairs in metal-doped TiO2, WO3, and Fe2O3 respectively form a hole in high oxidation potential but electron in low reduction potential, and those in nitrogen-doped TiO2, GaP, GaAs, CdS, and CdSe respective have an electron with high reduction potential but hole of low oxidation potential, resulting in the redox capabilities of the visible induced photocatalysts being worse than those of TiO2 photocatalyst.